I've had issue with the heater in my Mercury Sable for a number of years. Over that time span I've worked on it several times, with a range of results.

History

~5 years ago, I had a garage look into the issue with the heat not working. They flushed the radiator. Things worked decently for a few weeks afterwards, but it soon returned to an unacceptable level of heat.

Over the past few years I've flushed the radiator myself. Each time it worked OK (not as good as when the dealership flushed it) but returned to an unacceptable level of heat.

Last winter I replaced the heater core. Afterwards, things seemed to work well for the remainder of the season.

About 3 months ago I flushed the heater core as well as the entire cooling system. I believe I got the heater core completely clean. Things were better for a while (maybe 40% of maximum heating) but have now returned to almost no heat coming out at all.

Notes

It takes a long time for the heat gauge to register as anything but cold, about 10-15 minutes.

Leaving the heater off for a while, then turning it on provides good heat for about 5 seconds, then it quickly loses heating power.

There doesn't seem to be a leak in the system, as the coolant is always full.

The coolant always seems dirty every time I check it.

After having replacing and subsequently flushing the heater core, I don't believe that's the issue.

1 Answer
1

It takes a long time for the heat gauge to register as anything but cold, about 10-15 minutes.

This is almost definitely a stuck open thermostat. The engine should reach operation temperature within 10 minutes, meaning middle of the gauge. You should see it start moving up within about 5 minutes

Thanks Larry, I'll see if I can pick up a new thermostat and replace soon. I've replaced it at least once in the past, but maybe I got a bad one, or it's since been corrupted.
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FreeAsInBeerJan 3 '14 at 17:43

Thanks Larry. I tore into it this weekend and it turns out the thermostat was stuck open. Replaced it and now it's operating about 85% as well as my '07 Accord.
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FreeAsInBeerJan 6 '14 at 14:19